Shrink films are polymer films which on application of heat shrink in one or both directions. They are widely used as packaging and casing materials for both large and small products (e.g. industrial pallets, bottles, magazines, etc), generally with thicker films being used for larger items and thinner films for smaller items.
At present, the most widely used material for shrink film production is low density polyethylene (LDPE), optionally blended with other polymers to achieve a desired balance of properties (e.g. stiffness) and cost. LDPE shrink films may also include a coextruded polypropylene layer to reduce the incidence of fusion of the LDPE layer to shrink film wrapped objects during the heat treatment to shrink the film. These commonly used LDPE or LDPE-rich shrink films however suffer from various problems. In particular, the shrink film has insufficient mechanical strength for many end uses, the occurrence of hole formation during the shrinking process is undesirably high, and the holding force of the shrink film is undesirably low.
Shrink film is produced by extrusion through an annular die with a pressure difference applied to blow the extruded cylinder into a film and achieve the desired orientation within the film, i.e. to build a stress into the cooled film. Heat treatment results in stress relaxation and, as a result, shrinkage. Most of the shrinkage occurs while the film is at its hottest (generally ca. 120-130° C.) during the heat treatment; however the film continues to shrink as it cools. These are referred to as the hot shrink and the cold shrink respectively and for a polymer to function adequately as the base material for a shrink film it must meet the different requirements (in terms of melt strength, cold strength and other mechanical properties) of the hot shrink, cold shrink and post-shrinkage stages.
We have now found that these requirements are particularly well met, particularly for relatively thin shrink films, by linear low density polyethylenes (LLDPE) having a high molecular weight distribution (MWD) and containing an ethylene homopolymer and an ethylene copolymer. (The term MWD refers to the ratio (Mw/Mn) between the weight average molecular weight (Mw) and the number average molecular weight (Mn) of a polymer).